Factor "E" involved in the formation of a DNA synthesizing complex appears to be associated with membranes. It can be released and activated by detergent. Commercial DNA polymerase I, although it contains only a single polypeptide, exists in two separable forms. Only one of these is involved in complex formation. The 5' yields 3' exonuclease portion of DNA polymerase I, which is essential for cell viability, is also essential for complex formation from DNA polymerase, when "E" is added.